Published on Web 04/22/2005
A 2D Solid-State NMR Experiment To Resolve Overlapping Aromatic Resonances of Thiophene-Based Nematogens Tanneru Narasimhaswamy,† Dong-Kuk Lee,† Kazutoshi Yamamoto,† Narayanasastri Somanathan,‡ and Ayyalusamy Ramamoorthy*,† Biophysics Research DiVision and Department of Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109-1055, and Polymer Laboratory, Central Leather Research Institute, Adyar, Chennai, 600 020, India Received February 23, 2005; E-mail:
[email protected] Thiophene-based molecules have been shown to be highly valuable in the design of organic electronics such as light emitting diodes, thin film transistors, photovoltaic cells, electrochromic cells, molecular electronic junctions, and biosensors.1-4 Another important application of thiophene-based molecules is polarized emission for which liquid crystalline property is a prerequisite.5 The key step in designing such molecules is to characterize not only the mesophase behavior but also the structural details at atomistic-level resolution. In this context, solid-state NMR spectroscopy is a powerful technique that can provide structural details pertaining to various phases of such materials.6-9 Despite the recognized importance of these molecules, there are no detailed NMR studies on them. One of the reasons could be due to the overlap of aromatic resonances in the chemical shift spectrum of the mesophase as these materials essentially contain several phenyl and thiophene rings. As a consequence, it is difficult to determine the geometry and dynamics of the aromatic rings. Since the core fragment is the heart of the mesogen, it is important to develop new approaches to overcome these difficulties. In this study, for the first time, we demonstrate the feasibility of resolving aromatic resonances using a 2D solidstate NMR experiment on a thiophene mesogen and measuring heteronuclear dipolar couplings in the liquid crystalline phase without isotopic labeling. The molecular structure of a thiophene mesogen 4-{[thien-3ylmethylene]amino}phenyl-4-butoxybenzoate (TMAPB)10 is shown in Figure 1. The 13C chemical shift spectrum of TMAPB obtained at 130 °C (TCN ) 124 °C; TNI ) 153.8 °C) is also given in Figure 1. While the narrow spectral lines in Figure 1 indicate that the molecule is well-aligned in the external magnetic field, the only resonances that are not well-resolved are from the methine carbons of phenyl rings and the thiophene ring (138-162 ppm). In addition, the spectral resolution decreases when the temperature is increased as the span of chemical shift anisotropy as well as the degree of alignment decrease with the increasing temperature. Such a lowresolution spectrum often hampers further characterization of the molecule, particularly the core fragment of the nematogen that decides the liquid crystalline property. To overcome this difficulty, a 2D separated local field (SLF) experiment that correlates the 13C chemical shift and 1H-13C dipolar coupling was carried out. In recent years, 2D SLF experiments have been increasingly used to measure heteronuclear dipolar couplings from liquid crystalline samples to determine the orientational order.11-13 However, these 2D techniques typically use high radio frequency (rf) power (about several hundred watts for 13C and >1 kW for 15N), which can increase the sample temperature due to rf heating.14,15 Increase in the sample temperature alters the orientation of the molecule in the mesophase and could also cause a phase † ‡
University of Michigan. Central Leather Research Institute.
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Figure 1. Molecular structure and the nematic phase 13C chemical shift NMR spectrum of TMAPB at 130 °C. The spectrum was acquired using a Chemagnetics/Varian Infinity 400 MHz solid-state NMR spectrometer and a 5-mm triple resonance MAS Chemagnetics probe. A 50 kHz rf field strength for CP, ∼75 kHz TPPM decoupling of protons, 1-ms contact time for cross polarization, 612 scans, and a recycle delay of 3 s were used. The chemical shift assignment of ring A carbons was done by comparing with the nematic phase spectrum of 4-butoxy benzoic acid (spectra given in the Supporting Information). Since the phenyl rings undergo rapid reorientation along the para-axis, the ortho- and meta-carbons are chemically equivalent.
change. To overcome this problem, we have recently developed a low rf power 2D NMR technique: polarization inversion timeaveraged nutation spin exchange at the magic angle (PITANSEMA).16,17 It suppresses homonuclear dipolar coupling among protons and provides very high-resolution heteronuclear dipolar coupling spectra that can be used to measure the orientational order of an aligned molecule. In this study, we demonstrate the utility of this technique to resolve the overlapping aromatic resonances of TMAPB. A 2D PITANSEMA spectrum of TMAPB is given in Figure 2. The 13C chemical shift region (138-162 ppm) that showed poor resolution in Figure 1 is now well-resolved in the 2D spectrum. Even the C5-H signal from the azomethine linking unit that is overlapping with the quaternary carbon of the aromatic ring in the 1D spectrum at 193.4 ppm (Figure 1) is clearly resolved in the 2D spectrum. This higher resolution is provided by the narrow lines in the second frequency (i.e., the dipolar coupling) dimension of the 2D spectrum. In addition, the resolution is enhanced by suppressing signals from quaternary carbons as they do not have a nearby proton. In the 2D spectrum, there are seven peaks corresponding to seven different C-H carbons of phenyl and thiophene 10.1021/ja051160j CCC: $30.25 © 2005 American Chemical Society
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Figure 2. 2D PITANSEMA spectrum of TMAPB at 130 °C. An experimentally measured scaling factor of 0.6 was used. τ1 ) 10 µs, τ2 ) 15 µs, 128 t1 increments, 16 scans, and a 3-s recycle delay were used.
0.46 (150 °C) for phenyl ring B, and 0.60 (130 °C) and 0.50 (150 °C) for the (azomethine) linking unit. These order parameters are consistent with the nematic phase of the sample. The measured dipolar couplings and the determined order parameters decreased with the increasing temperature and became zero at the clearing temperature (153.8 °C). Since the crystal structure of the molecule is unknown, by using the order parameter of ring B that is linked to thiophene, we determined the orientations of the thiophene C-H bonds from the molecular axis. The angles between the molecular axis and the methine bonds in the thiophene ring are 37 ( 2°, 40 ( 2°, and 43 ( 2° for H-C1, H-C3, and H-C4, respectively. The approach demonstrated in this study can be extended to the characterization of banana and other types of liquid crystals. In addition, 3D organization of the molecules can be determined as shown for biomacromolecules.19 In view of the molecular kink between the central phenyl ring and the side phenyl rings in the banana mesogen, the C-H dipolar couplings that differ in magnitude can in principle be used to determine the geometry and bent angle. Acknowledgment. This work was partly supported by funds from NIH (AI054515 to A.R.). T.N. is on sabbatical leave from CLRI, Chennai, India.
Figure 3. 13C-1H dipolar coupling slice obtained from Figure 2 at 144.4 ppm shows dipolar coupling doublets for 1H-13C8 (inner; 2.4 kHz) of ring B and 1H-13C4 (outer; 4.4 kHz) of thiophene ring.
rings. A C-H dipolar coupling slice taken from the 2D spectrum (Figure 2) contains two dipolar couplings corresponding to H-C4 and H-C8 and is shown in Figure 3. Since the molecular axis is nearly collinear with the para-axis of the phenyl rings, the smaller dipolar coupling value is assigned to the H-C8 bond. Similarly, the four different pairs of peaks in the 2D spectrum with a dipolar coupling (2.2-2.4 kHz belong to the phenyl ring methine carbons, while the three pairs of peaks in the region (3.6-5.5 kHz originate from the thiophene C-H groups. Since 4-butoxy benzoic acid is the starting material for the synthesis and it represents ring A of TMAPB, we utilized the PITANSEMA spectrum of 4-butoxy benzoic acid (data given in the Supporting Information) as a reference to determine the molecular axis of TMAPB. The C-H dipolar couplings of the phenyl rings measured from TMAPB (Figure 2) and 4-butoxy benzoic acid are similar. Therefore, it is reasonable to assume that the molecular axis of TMAPB is nearly collinear with the para-axis of the phenyl rings as in 4-butoxy benzoic acid. Since the order parameters of phenyl and thiophene rings cannot differ significantly, the difference in the dipolar coupling values (Figure 2) clearly suggests that the orientation of the thiophene ring significantly differs from that of phenyl rings in TMAPB. This is because the substitution at the third position of thiophene (with azomethine) deviates the orientation of the ring from the molecular axis, while the 2,5-substitution of thiophene makes the ring part of the molecular axis.18 The measured dipolar couplings are used to determine the order parameters (S) using D ) -S(hγCγH/4π2rCH3)(3 cos2 θ - 1)/2; where D is the measured C-H dipolar coupling and θ is the angle between the C-H vector and the external magnetic field axis. The order parameters are 0.51 (130 °C) and 0.44 (150 °C) for phenyl ring A, 0.53 (130 °C) and
Supporting Information Available: Molecular structure, 13C chemical shift spectrum, and a 2D PITANSEMA spectrum of 4-butoxy benzoic acid. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Liu, J.; Kadnikova, N. E.; Liu, Y.; McGehee, M. D.; Frechet, J. M. J. J. Am. Chem. Soc. 2004, 126, 9486-9487. (2) Janzen, D. E.; Burnand, W. M.; Ewbank, P. C.; Pappenfus, T. M.; Higuchi, H.; da Silva Filho, D. A.; Young, V. G.; Bredas, J.-L.; Mann, K. R. J. Am. Chem. Soc. 2004, 126, 15295-15308. (3) Rajca, A.; Miyasaka, M.; Pink, M.; Wang, H.; Rajca, S. J. Am. Chem. Soc. 2004, 126, 15211-15222. (4) See, for example, the special issue on organic electronics and references therein: Jenekhe, S. A. Chem. Mater. 2004, 16, 4381-4382. (5) (a) O’Neill, M.; Kelly, S. M. AdV. Mater. 2003, 15, 1135-1146. (b) Grell, M.; Bradely, D. C. D. AdV. Mater. 2003, 11, 895-905. (6) Burnell, E.; De Lange, C. A. NMR of Ordered Liquids; Kluwer: Boston, 2003. (7) Ramanathan, K. V.; Sinha, N. Monatsh. Chem. 2002, 133, 1535-1548. (8) Fischbach, I.; Pakula T.; Minkin, P.; Fechtenko1tter, A.; Mu1llen, K.; Spiess, H. W. J. Phys. Chem. B 2002, 106, 6408-6418. (9) Percec, V.; Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanovskaya, L.; Singer, K. D.; Balagurusamy, V. S. K.; Heibey, P. A.; Schnell, I.; Rapp, A.; Spiess, H. W.; Hudson, S. D.; Duan, H. Nature 2002, 419, 384-862. (10) Narasimhaswamy, T.; Somanathan, N.; Lee, D. K.; Ramamoorthy, A. Chem. Mater. 2005, 17, 2013-2018. (11) Dvinskikh, V. S.; Zimmermann, H.; Maliniak, A.; Sandstrom, D. J. Magn. Reson. 2003, 163, 46-55. (12) Zimmermann, H.; Bader, V.; Poupko, R.; Wachtel, E. J.; Luz, Z. J. Am. Chem. Soc. 2002, 124, 15286-15301. (13) Caldarelli, S.; Hong, M.; Emsley, L.; Pines, A. J. Phys. Chem. 1996, 100, 18696-18701. (14) Ramamoorthy, A.; Wu, C. H.; Opella, J. S. J. Magn. Reson. 1999, 140, 131-140. (15) Ramamoorthy, A.; Wei, Y.; Lee, D. K. Annu. Rep. NMR Spectrosc. 2004, 52, 2-52. Yamamoto, K.; Lee, D. K.; Ramamoorthy, A. Chem. Phys. Lett. 2005, 407, 289-293. (16) Lee, D. K.; Narasimhaswamy, T.; Ramamoorthy, A. Chem. Phys. Lett. 2004, 399, 359-362. (17) Nishimura, K.; Naito, A. Chem. Phys. Lett. 2005, 402, 245-250. (18) (a) Eichhorn, S. H.; Paraskos, A. J.; Keiki, K.; Swager, T. M. J. Am. Chem. Soc. 2002, 124, 12742-12751. (b) Seed, A. J.; Cross, G. J.; Toyne, K. J.; Goodby, J. W. Liq. Cryst. 2003, 30, 1089-1107. (19) (a) Marassi, F. M.; Opella, S. J. J. Magn. Reson. 2000, 144, 150-155. (b) Wang, J.; Denny, J.; Tian, C.; Kim, S.; Mo, Y.; Kovacs, F.; Song, Z.; Nishimura, K.; Gan, Z.; Fu, R.; Quine, J. R.; Cross, T. A. J. Magn. Reson. 2000, 144, 162-167. (c) Mascioni, A.; Eggimann, B. L.; Veglia, G. Chem. Phys. Lipids 2004, 132, 133-144.
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